The concentration of atmospheric carbon dioxide (CO2) continues to rise, as shown by, for example, IPCC, Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007 [Core Writing Team, Pachauri, R. K and Reisinger, A. (eds.)], IPCC, Geneva, Switzerland, 104 pp. The concentration of atmospheric CO2 is rising at the rate of approximately 2 parts per million per year (ppm/yr). The concentration of CO2 in the atmosphere is approximately 385 ppm.
The world's oceans have been absorbing and releasing atmospheric CO2 for eons. Atmospheric CO2 dissolves in the oceans' water, reacting with the seawater to form carbonic acid. Carbonic acid in turn releases hydrogen ions (H+), forming bicarbonate (HCO3−) and carbonate (CO3−2) ions. The pH of seawater, which determines the relative fractions of dissolved CO2, HCO3− and CO3−2, is typically around 8.3, meaning that most of the dissolved total carbon in seawater is in the form of HCO3−, as discussed in, for example, James N. Butler, Carbon Dioxide Equilibria and Their Applications, Addison-Wesley Publishing Company, Menlo Park, Calif., 1982. As the atmospheric concentration of CO2 increases, so too does the oceanic concentration of dissolved CO2 increase. See, e.g., Holli Riebeek, The Ocean's Carbon Balance, NASA Earth Observatory Feature Article, http://earthobservatory.nasa.gov/Features/OceanCarbon/printall.php (last visited Jun. 20, 2011). The volumetric concentration of CO2 in seawater is comparatively much higher than that in the atmosphere, with approximately 100 times as much CO2 in one liter of seawater as there is in one liter of air.
Techniques for separating CO2 from streams of mixed gases, such as separating CO2 from the atmosphere, typically involve a two-step process of capture and desorption/regeneration. First, the gas is contacted with an aqueous “pre-capture solution” that reacts with the CO2 gas in the mixed-gas stream, “capturing” the CO2 into what is then referred to as a “post-capture solution.” A stream of pure CO2 gas can then be desorbed from the CO2-rich aqueous post-capture solution, while at the same time regenerating the post-capture into a pre-capture solution that can be reused for additional capture cycles.
The pre-capture solution is contained in a “contactor,” a structure that contacts the mixed-gas stream from which the CO2 is to be separated with the pre-capture solution. Various pre-capture solutions exist, including aqueous hydroxide pre-capture solutions such as potassium hydroxide (KOH) or sodium hydroxide (NaOH); aqueous carbonate pre-capture solutions such as potassium carbonate (K2CO3) or sodium carbonate (Na2CO3); and aqueous bicarbonate pre-capture solutions such as potassium bicarbonate (KHCO3) or sodium bicarbonate (NaHCO3). Other pre-capture solutions are known, for example, monoethanolamine (MEA), which is used in gas-stream scrubbing applications to remove, for example, CO2 from flue gas. The capture of CO2 gas into hydroxide/carbonate/bicarbonate pre-capture solutions converts the original pre-capture solution into a more acidic post-capture solution consisting of a mixture of hydroxide (KOH or NaOH), carbonate (K2CO3 or Na2CO3), and/or potassium bicarbonate (KHCO3) or sodium bicarbonate (NaHCO3) post-capture solutions, as examples.
After CO2 capture and desorption/regeneration, the post-separation CO2 can be, for example, geologically sequestered, or incorporated into useful products such as concrete, as shown by Calera, Green Cement for a Blue Planet, http://dev.calera.com/index.ppp/technology/technology_vision/index.html (last visited Jun. 20, 2011); plastics, as shown by G. A. Olah et al., Beyond Oil and Gas: The Methanol Economy, Wiley-VCH (2006); or liquid hydrocarbon fuels, as shown by F. S. Zeman & D. W. Keith, Carbon Neutral Hydrocarbons, Phil. Trans. R. Soc. A, 366, 3901-3918 (2008), and PARC, Energy Efficiency, http://www.parc.com/work/focus-area/adaptive-energy/ (last visited Jun. 20, 2011).
Generation of liquid hydrocarbon fuel, such as gasoline, diesel, or JP-8, from CO2 separated from mixed-gas streams may be of particular importance in remote field operations, such as those engaged in by the U.S. military (see, for example, JASON, “Reducing DOD Fossil-Fuel Dependence,” JSR-06-135, 2006, p. 30) or the U.S. NSF Antarctic Program (see, for example, J. Swift, et. al., “Report of the Subcommittee on US Antarctic Program Resupply,” NSF Office of Polar Programs Advisory Committee, Arlington, Va., 2005). However, due to the large size of contactors needed for traditional CO2 separation, the remote, on-site generation of liquid hydrocarbon fuel has so far not been realized.
Bipolar membrane electrodialysis (BPMED) can be used to convert aqueous salt solution into acids and bases without the addition of other chemicals. BPMED devices use ion exchange membranes to separate ionic species in solution when a voltage is applied across a stack of membranes. BPMED of aqueous carbonate solutions at pressures above ambient pressure has been shown to efficiently desorb CO2 gas from post-capture solutions. See U.S. patent application Ser. No. 12/969,465; U.S. patent application Ser. No. 12/269,485. Because of the very low (385 ppm) concentration of CO2 in the atmosphere, large volumes of air must be processed to capture CO2 from the atmosphere into aqueous carbonate solution systems. This results in a system that may not be as compact as required for certain applications, such as deployment in remote locations. Because the surface of the ocean essentially serves as a contactor, CO2 desorption directly from seawater eliminates the need for contactors, resulting in a more compact system for CO2 separation.
Example embodiments address these and other disadvantages.